A device for regulating the boost pressure of an internal combustion engine having two exhaust-gas turbochargers is known from German Patent No. DE 19810174. In this context, the mass flow rate of air drawn in is measured by an air-mass flow sensor in each induction pipe in which a compressor of a turbocharger is situated. In addition, means are provided that calculate a controlled variable from the air-mass flow rates measured in the two induction pipes, a controller deriving a correcting variable for the turbines of the two turbochargers from the controlled variable. A comparator compares the two measured air-mass flow rates to each other, in order to monitor the synchronous operation of the two turbochargers. If, for example, one of the turbochargers generates a higher boost pressure than the other turbocharger, then the comparator generates a correcting variable for the bypass valve of the former turbocharger, which will allow the bypass flow rate of the turbine of the former. turbocharger to be increased. On the contrary, if the latter turbocharger generates a higher boost pressure than the former turbocharger, then the comparator generates a correcting variable for the bypass valve of the latter turbocharger, which will allow the bypass flow rate of the turbine of the latter turbocharger to be increased. In this manner, the use of two air-mass flow sensors in separate induction pipes of the two turbochargers allows simple synchronous regulation to be implemented.
German Patent Application No. DE 19615033 describes a set-up for detecting variations in the speed of two exhaust-gas turbochargers. In this context, the difference between the pressures prevailing in the two exhaust tracts of the two cylinder banks, upstream from the exhaust-gas turbines, is ascertained, and a relevant speed difference is signaled when the ascertained pressure difference exceeds a specifiable threshold value.
Furthermore, exhaust-gas turbochargers having a variable turbine geometry are well-known. In this context, an actuator for controlling the boost pressure generated by a compressor of the exhaust-gas turbocharger is triggered. This actuator is not, as described above, a bypass valve in a bypass which circumvents the turbine of the exhaust-gas turbocharger or the compressor of the exhaust-gas turbocharger, but rather an actuator for varying the geometry of the turbine of the turbocharger.
Exhaust-gas turbochargers having such a variable turbine geometry are currently widely used to significantly increase the power of the internal combustion engine. Compared to conventional exhaust-gas turbochargers having a bypass valve, the variable turbine geometry particularly allows for an improvement in the response in the event of a positive load change, i.e. a load change in the direction of a higher load as occurs, for example, in a starting process. In this context, the boost pressure is regulated by adjusting the guide vanes in the turbine of the turbocharger, using a separate actuator. The consequently changed speed of the shaft connecting the turbine and the compressor of the exhaust-gas turbocharger produces a higher or lower boost pressure on the side of the compressor.